Cell proliferation and programmed cell death play important roles in the growth and development of an organism. In proliferative diseases such as cancer, the processes of cell proliferation and/or programmed cell death are often perturbed. For example, a cancer cell may have unregulated cell division through either the overexpression of a positive regulator of the cell cycle or the loss of a negative regulator of the cell cycle, perhaps by mutation. Alternatively, a cancer cell may have lost the ability to undergo programmed cell death through the overexpression of a negative regulator of apoptosis. Therefore, there is a need to develop new therapeutic agents that will restore the processes of checkpoint control and programmed cell death to cancerous cells.
RNA interference (RNAi) is an evolutionarily conserved process in which recognition of double-stranded RNA (dsRNA) ultimately leads to posttranscriptional suppression of gene expression. In particular, RNAi induces specific degradation of mRNA through complementary base pairing between the dsRNA and the target mRNA. In several model systems, this natural response has been developed into a powerful tool for the investigation of gene function (see, e.g., Elbashir et al., Genes Dev., 15:188-200 (2001); Hammond et al., Nat. Rev. Genet., 2:110-119 (2001)).
RNAi is generally mediated by short dsRNAs such as small interfering RNA (siRNA) duplexes of 21-23 nucleotides in length or by longer Dicer-substrate dsRNAs of 25-30 nucleotides in length. Unlike siRNAs, Dicer-substrate dsRNAs are cleaved by Dicer endonuclease, a member of the RNase III family, to produce smaller functional 21-mer siRNA duplexes. The 21-mer siRNA (whether synthesized or processed by Dicer) recruits the RNA-induced silencing complex (RISC) and enables effective gene silencing via sequence-specific cleavage of the target sequence.
Although the precise mechanism is still unclear, RNAi provides a powerful approach to downregulate or silence the transcription and translation of a gene of interest. In particular, for the treatment of neoplastic disorders such as cancer, RNAi may be used to modulate (e.g., reduce) the expression of certain genes, e.g., an anti-apoptotic molecule, a growth factor, a growth factor receptor, a mitotic spindle protein, a cell cycle protein, an angiogenic factor, an oncogene, an intracellular signal transducer, a molecular chaperone, and combinations thereof.
However, a safe and effective nucleic acid delivery system is required for RNAi to be therapeutically useful. Viral vectors are relatively efficient gene delivery systems, but suffer from a variety of limitations, such as the potential for reversion to the wild-type as well as immune response concerns. Furthermore, viral systems are rapidly cleared from the circulation, limiting transfection to “first-pass” organs such as the lungs, liver, and spleen. In addition, these systems induce immune responses that compromise delivery with subsequent injections. As a result, nonviral gene delivery systems are receiving increasing attention (Worgall et al., Human Gene Therapy, 8:37 (1997); Peeters et al., Human Gene Therapy, 7:1693 (1996); Yei et al., Gene Therapy, 1:192 (1994); Hope et al., Molecular Membrane Biology, 15:1 (1998)).
Complexes of nucleic acid and cationic liposomes (i.e., lipoplexes) are a commonly employed nonviral gene delivery vehicle. For instance, lipoplexes made of an amphipathic compound, a neutral lipid, and a detergent for transfecting insect cells are disclosed in U.S. Pat. No. 6,458,382. Lipoplexes are also disclosed in U.S. Patent Publication No. 20030073640. However, lipoplexes are large, poorly defined systems that are not suited for systemic applications and can elicit considerable toxic side-effects (Harrison et al., Biotechniques, 19:816 (1995); Li et al., The Gene, 4:891 (1997); Tam et al, Gene Ther., 7:1867 (2000)). As large, positively charged aggregates, lipoplexes are rapidly cleared when administered in vivo, with highest expression levels observed in first-pass organs, particularly the lungs (Huang et al., Nature Biotechnology, 15:620 (1997); Templeton et al., Nature Biotechnology, 15:647 (1997); Hofland et al., Pharmaceutical Research, 14:742 (1997)).
Other liposomal delivery systems include, for example, the use of reverse micelles, anionic liposomes, and polymer liposomes. Reverse micelles are disclosed in U.S. Pat. No. 6,429,200. Anionic liposomes are disclosed in U.S. Patent Publication No. 20030026831. Polymer liposomes that incorporate dextrin or glycerol-phosphocholine polymers are disclosed in U.S. Patent Publication Nos. 20020081736 and 20030082103, respectively. However, such liposomal delivery systems are unsuitable for delivering nucleic acids such as interfering RNA to tumors because they are not of the desired size (i.e., less than about 150 nm diameter), are not preferentially delivered to tumor sites, and do not remain intact in the circulation for an extended period of time in order to achieve delivery to tumor sites. Rather, effective intracellular delivery of nucleic acids such as interfering RNA to tumors requires a highly stable, serum-resistant nucleic acid-containing particle that preferentially targets tumors such as solid tumors and does not interact with cells and other components of the vascular compartment.
Thus, there remains a strong need in the art for novel compositions and methods for preferentially introducing nucleic acids such as interfering RNA into tumor cells. In addition, there is a need in the art for methods of downregulating the expression of genes associated with tumorigenesis or cell transformation to treat or prevent cancer. The present invention addresses these and other needs.